A polymer having π conjugated double bonds represented by polyacetylene, polythiophene, polyaniline, polypyrrole and the like is known to be an electric conductor (electrically conductive polymer) by doping with a donor or an acceptor, and its application to an antistatic agent, a solid electrolyte of a capacitor, an electrically conductive coating material, an electrochromic device, a transparent electrode, a transparent electrically conductive film, a chemical sensor, an actuator, etc. has been studied. Heretofore, an electrically conductive polymer is insoluble and infusible and is thereby problematic in formability, and to dissolve it, a polar organic solvent (for example, an amido solvent) with a heavy environmental burden is necessary. Thus, a water-soluble and easily formable electrically conductive polymer, which is soluble in water with a light environmental burden, has been desired.
In recent years, as an electrically conductive polymer, a polythiophene called poly(3,4-ethylenedioxythiophene) (PEDOT) has been actively studied (for example, Patent Document 1), however, it is known that since 3,4-ethylenedioxythiophene (EDOT) as its material monomer is hardly soluble in water (2.1 g/L water, 0.2 wt %), the obtainable electrically conductive polymer is insoluble in water.
Accordingly, in order to obtain a water-soluble electrically conductive polymer, a method has been proposed in which EDOT is polymerized in the presence of a water-soluble high molecular weight dopant such as polystyrene sulfonic acid (PSS) (for example, Patent Document 2, called PEDOT-PSS).
Patent Document 2 discloses that the polymer becomes water-soluble and has improved formability by polyanions being incorporated both as a dopant and as a water dispersing agent. However, the electrically conductive polymer disclosed in Patent Document 2 has problems such that its electrical conductivity is low since it contains a large quantity of polymer moieties with low electrical conductivity, which do not contribute to doping, it has low heat resistance and water resistance since it has sulfo groups in large excess, and an apparatus may be eroded by strong acidity.
By the way, since the electrically conductive polymer disclosed in Patent Document 2 has both favorable electrical conductivity and formability, its application to a solid polymer electrolyte of a capacitor and printable electronics has been desired.
Examples of the former include a solid electrolyte of an aluminum solid electrolytic capacitor, and a high capacity and a low ESR (equivalent series resistance) as the capacitor performance are to be achieved.
Further, in the case of the latter, for example, when technique of e.g. an inkjet is applied, if the particle size of the electrically conductive polymer in the aqueous solution is large, problems such as clogging of a nozzle may arise.
On the other hand, another method to obtain a water-soluble electrically conductive polymer proposed may be such that a compound having substituents having both a function to impart water solubility and a doping function (for example, sulfo groups or sulfonate groups) introduced in a polymer molecular chain by covalent bonds directly or via a spacer, is polymerized to obtain a water-soluble self-doping electrically conductive polymer which is excellent in the formability (for example, Patent Documents 3 and 4, Non-Patent Documents 1 and 2). Among them, poly(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-lymethoxy)-1-butanesulfonic acid) (PEDT-S) substituted by a straight-chain alkylene sulfonic acid group is reported to have a high electric conductivity of 10-30 S/cm (Patent Document 5, Non-Patent Documents 2 and 3).
As an example of application of a water-soluble self-doping electrically conductive polymer disclosed in such documents, application to an antistatic film-forming material of a resist used at the time of forming a circuit pattern of a semiconductor by electron lithography may be mentioned. This application is thanks to advantageous such that the electrically conductive polymer is water-soluble and is thereby less likely to damage a lipid-soluble resist, and washing with water can be carried out after exposure (for example, Non Patent Document 4). However, along with high integration of a semiconductor in recent years, finer pattern formation is required, and therefore a polymer having a higher electrical conductivity (antistatic performance) has been desired.
Accordingly, a self-doping water-soluble electrically conductive polymer which can impart processability without adding other components which do not contribute to improvement of the electrical conductivity to make the polymer be water-soluble, which has favorable water solubility and electrical conductivity and in addition, which has a sufficiently small particle size of the polymer when formed into an aqueous solution, has been desired.